A former Donald Trump campaign adviser was secretly arrested in July and has pleaded guilty to charges of lying to the FBI about contacts with Russia during the 2016 campaign, according to court documents unsealed by special counsel Robert Mueller on Monday.

The former Trump foreign policy adviser, George Papadopoulos, admitted to making “numerous” false statements to the FBI about his repeated efforts to arrange an “off the record” meeting between Trump campaign officials and Russian President Vladimir Putin’s office. He is now cooperating with Mueller’s investigation, according to the unsealed court records.

Among the subjects he is providing information on, according to the court records, are his communications with an unidentified Russian professor in London with close ties to the government in Moscow, who informed him in April 2016 that the Russians had “dirt” on Hillary Clinton, including “thousands of emails.”

The charges against Papadopoulos provide substantial new details about communications between the Trump campaign and figures close to the Russian government — a central part of the investigation by Mueller. The court records unsealed Monday do not provide a clear account of how senior Trump officials followed up on Papadopoulos’s efforts to set up a meeting with Russian officials.

But they quote one unidentified campaign “supervisor” as emailing him in August 2016 that “I would encourage you” to make a trip to Moscow to arrange such a meeting. A Trump campaign source identified the supervisor as Sam Clovis, a conservative radio host who was co-chairman of the campaign. Another “high ranking” official — identified by the source as campaign chairman Paul Manafort — received an email from Papadopoulos saying that “Russia has been eager to meet Mr. Trump for quite some time and has been reaching out to me to discuss.” Manafort forwarded that email to his associate Rick Gates and wrote: “Let’s discuss. We need someone to communicate that DT is not doing these trips. It should be someone low level in the campaign so as not to send any signal.” (Some of these emails were quoted in a Washington Post story this past August that first identified Clovis, Manafort and Gates as the campaign officials who sent and received them.)

Papadopoulos admitted lying to the FBI about his Russian contacts when he was initially interviewed in January, and, according to the “statement of offense” unsealed Monday, by doing so “impeded the FBI’s ongoing investigation into the existence of any links or coordination between individuals associated with the [Trump] campaign and the Russian government’s efforts to interfere with the 2016 presidential election.”

Trump first mentioned Papadopoulos’s role in the campaign during a meeting with the Washington Post editorial board in March 2016, identifying him as one of five foreign policy advisers who had joined his team. Papadopoulos immediately drew attention because of his apparent lack of foreign policy experience: A 2009 college graduate, he had worked as an intern and researcher at the Hudson Institute, a conservative think tank, and listed as one of his credentials on his LinkedIn profile his participation in a Model U.N. program for students.

It is far from clear how much influence, if any, he wielded on Trump’s campaign. But the charges laid out by Mueller flesh out what U.S. intelligence officials have long said was a concerted effort by Moscow to cultivate figures close to the Trump campaign — in part by offering damaging information about Clinton. “They set out what appears to be a classic Russian intelligence operation, in which a foreign policy adviser for the Trump campaign was approached — or bumped, in intelligence parlance — by a person claiming to have substantial connections to Russian officials,” said Rep. Adam Schiff, D-Calif., the ranking Democrat on the House Intelligence Committee.

When Papadopoulos joined the Trump campaign in early March 2016, he “understood that a principal foreign policy focus of the campaign was an improved U.S. relationship with Russia,” according to the documents.

A week later, while traveling in Italy, Papadopoulos met a Russian professor based in London who claimed to have “substantial connections” with Russian government officials, the statement says. After learning about his role in the Trump campaign, the professor “appeared to take great interest” in Papadopoulos and later introduced him to a Russian woman whom Papadopoulos described in an email as “Putin’s niece.” (He later learned she wasn’t.) Papadopoulos told the two Russians that he had “connections that could help arrange a meeting between then candidate Trump and President Putin,” the statement says.

That meeting never took place, but key Trump advisers including Donald Trump Jr., campaign manager Manafort — whose indictment on unrelated charges was also unsealed Monday — and Trump son-in-law Jared Kushner met at Trump Tower with Russian officials offering compromising material about Hillary Clinton on June 9. It is unclear whether Papadopoulos played any role in setting up that meeting, which is believed to be a focus of Mueller’s probe.

Papadopoulos met the Russian professor on April 26, 2016. It was at this meeting that the professor said he had just returned from a trip to Moscow, where he met with “high level Russian government officials.” The professor told Papadopoulos that the Russians had obtained “dirt” on Clinton, that “the Russians had emails of Clinton” and that “they have thousands of emails.”

After this meeting, Papadopoulos stepped up his efforts to arrange meetings between Trump officials and the Russians. “Have some interesting messages coming in from Moscow about a trip when the time is right,” he emailed a senior policy adviser to Trump the next day, April 27. He emailed another “high ranking campaign official” the same day about “Russia’s interest in hosting Mr. Trump. Have been receiving a lot of calls over the last month about Putin wanting to host him and the team when the time is right.” He also emailed the professor three days later, on April 30, thanking him for his “critical help” in seeking to arrange a meeting between the campaign and Russian officials, adding: “It’s history making if it happens.”

A photo published on Donald Trump’s Instagram of George Papadopoulos (circled by Yahoo News) at a Trump campaign national security meeting in March 2016.

As laid out in the statement of offense, Papadopoulos continued to stay in touch with the professor about setting up a meeting and peppered campaign officials with updates about his efforts. On June 19, 2016, just 10 days after the Trump Tower meeting, Papadopoulos emailed a high-ranking Trump campaign official with the subject line “New message from Russia.”

“The Russian ministry of foreign affairs messaged and said that if Mr. Trump is unable to make it to Russia, if a campaign rep (me or someone else) can make it for meetings? I am willing to make the trip off the record if its in the interest of Mr. Trump and the campaign to meet specific people.”

It was after several weeks of communications about this “off the record” meeting that the unidentified Trump campaign supervisor appeared to give the green light: “I would encourage you” and another foreign policy adviser to “make the trip if it is feasible.”

When first questioned by the FBI on Jan. 27, Papadopoulos acknowledged meeting with the Russian professor and hearing about Russian “dirt” on Clinton, but he insisted his communications all took place before he joined the Trump campaign. “I wasn’t even on the Trump team, that wasn’t even on the radar. … This was a year ago, this was before I even got with Trump,” he told them, falsely.

After his FBI interview, Papadopoulos took further steps to conceal his campaign communications by deactivating a Facebook account that contained information about those communications. On July 27, returning to the country from a foreign trip, he was arrested by the FBI at Dulles International Airport, and in subsequent questioning began to “provide information and answer questions,” the statement of offense reads.